Meiotic chromosome associations in homothallic strains of S. cerevisiae were investigated by analysis of meiotic spore colonies and crossing over in +/+/− trisomics. The segregants of these aneuploids produce phenotypically distinguishable tetrasomic spore colonies. The data indicate that trivalent associations occur with a high frequency in trisomics of chromosome XVII and that the frequency of second division segregation is markedly increased over that found in the normal dipoid.

Maternal inheritance of extranuclear mitochondrial genes has been demonstrated in Aspergillus nidulans using the ‘blue’ ascospore colour mutants in combination with heterokaryon incompatible strains. It appears that heterokaryosis is not a prerequisite of sexual outcrossing, and that recombination of extranuclear mitochondrial markers does not occur in the sexual stage of the cell cycle.

The Lac plasmids in two Klebsiella strains, V9A and RE1544, give characteristic Lac-permease defective mutations which produce a lactose-negative (ML−) phenotype in the host strain, in spite of the presence of a lac operon on the chromosome. These ML− clones can revert to the wild-type (ML+) phenotype, and the mechanism of this reversion is examined. In V9A carrying its own Lac plasmid (FKlac), it is shown that reversion of ML− to ML+ is usually the result of mutation to constitutivity of one of two galactoside permeases which are not induced by lactose but accumulate lactose when otherwise induced or made constitutive. However, in one out of the 51 ML+ revertants tested the mechanism of reversion appeared to be a change back to wild type of the Lac plasmid's own permease gene. In V9A carrying the Lac plasmid (pRE6) from RE1544, successive changes of phenotype were obtained of ML+ to ML− to ML+ to ML− to ML+; these were all found to be the result of changes in the plasmid permease gene, and could be simply explained if an IS sequence could insert and excise from this gene.

Four inbred strains of mice were used, differing in the total percentages of spermatozoa with abnormal heads (KE, 22·1%; C57, 26·4%; KP, 7·7%; CBA, 5·5%) and in the frequency distribution of abnormality types, as divided into four arbitrary classes. The most variable class 2 (narrow heads with canals inside the nuclear material) accounted for 47% of all abnormalities in KE strain, was common in CBA (29%) and almost missing in KP and C57 strains. F1 hybrids from the diallel crosses of these strains exhibited highly significant heterosis effects and significant reciprocal differences in the total percentage of abnormalities. The relative frequency of class 2 ranked in F1 hybrids in a similar order as calculated from the mid-parental values. After seven generations of backcrosses performed to introduce the Y chromosome from CBA to the genetical background of the KE strain, the total percentage of abnormalities was significantly reduced, although the relative proportion of class 2 was similar to that in KE strain. Also the Y chromosome from C57 strain, introduced into the genetical background of KE strain, caused a significant reduction of total abnormalities, but again the relative frequency of class 2 was not affected. It is concluded that the Y chromosome plays an important role in determining the total percentage of sperm head abnormalities, but does not seem to be involved in influencing specific abnormality types.

In devising maps of the positions of gene loci along chromosomes, by measurements of crossing-over, it is usual to assume that the chiasmata which give rise to the crossing-over are randomly located, unless there is evidence to the contrary. If chiasmata are in fact preferentially located at certain points or regions this not only affects mapping, but also is of interest for population genetics since it may favour the development during evolution of blocks of genes relatively undisturbed by crossing-over. Preferential localization of chiasmata is well known in a number of disparate organisms (Darlington 1965; John & Lewis, 1965), but has so far been little studied in mammals. Evidence is presented suggesting tha chiasmata are in fact non-randomly located in certain chromosomes of the mouse.

A metric character determined by a large number of loci without epistasis is normally distributed. In the absence of linkage the joint distribution in two or more relatives is multivariate normal, so that all regressions are linear and have constant residual variance. In the presence of linkage this is no longer true except in the case of parent and child; for all other types of relatives the regression line is unaffected by linkage but the residual variance about this line is no longer constant but increases away from the mean.

The yellow mutant (y − 1:0·0) of Drosophila melanogaster shows a reduction in body pigmentation associated with a decrement in locomotor activity and in male competitive mating ability. The effects of the mutant gene are specific to locomotor activity in the adult fly, measures of larval activity being unaffected. In the presence of active females yellow males offer a reduced intensity of sexual stimulation because they are less able to maintain contact during courtship. However, the impaired locomotor performance of yellow males is not the general cause for their reduced competitive mating ability, since the stimuli provided by yellow mutants courting inactive females appear to be both quantitatively and qualitatively indistinguishable from those of their wild-type male sibs. Nor is there any good evidence, as measured by the frequency of rejection responses, that the courtship stimulation offered by the yellow males is less acceptable to the females. The mutant males are nevertheless unsuccessful in achieving copulation with such females. It is suggested that impairment of mating ability in yellow males may be caused by changes in the efficiency for mating of their secondary sexual structures due to the effect of the yellow gene on the properties of the cuticle.

Baker (1971) presented genetic evidence that the inviability of In(1)scL8/O and scS1/O males in Drosophila melanogaster is due to position effect suppression of ribosomal RNA cistrons. Although scL8/O and scS1/O males are inviable, scL8/scL8 and scS1/scS1 females are viable. We therefore asked the following question: Is scL8/O or scS1/O viable when part of a gynandromorph? In other words, is position effect suppression of rRNA genes autonomous or non-autonomous in gynandromorphs? In this paper preliminary evidence is presented which suggests that position effect suppression of rRNA cistrons is non-autonomous. The evidence is that scLS/ or scS1/O (male) parts of gynandromorphs are not only viable but normal in appearance.

The resistance plasmid RP4 can be used to mediate conjugation in Acinetobacter calcoaceticus. Transfer of chromosomal genes occurs only on solid surfaces and not in liquid matings. Two RP4-carrying donor strains which donate the chromosome in different directions from different origins have been isolated. As a result it is possible to demonstrate that the linkage map of A. calcoaceticus is circular.

The fate of RP4 in recombinants was examined. Although frequent loss of the complete plasmid was detected, segregation of plasmid antibiotic resistances was not observed.

A possible mechanism by which RP4 could interact with the A. calcoaceticus chromosome is discussed.

An isolation method for N-methyl-N′-nitrosoguanidine-induced catalase negative mutants of P. shermanii based on replica plating is described. In contrast to previous methods, it extends to the early stages of tetrapyrrole biosynthesis which are common in both corrins and porphyrins. It may thus aid in elucidating the mechanism and control of porphyrin and corrin biosynthesis. Some preliminary results are discussed.

Lysogenization by phage P22 carrying amber mutations in genes 12 and 18

By Amiram Ronen and Myron Levine

Department of Genetics, The Hebrew University of Jerusalem, Israel, and the Department of Human Genetics,

The University of Michigan, Ann Arbor, Michigan 48109, U.S.A.

Vol. 27, No. 2, p. 337: add to Acknowledgements –

‘In addition, this research was supported by Grant No. 738 from the United States–Israel Binational Science Foundation (BSF), Jerusalem, Israel.’